This invention relates to a focusing detecting mechanism in a single-lens reflex camera in which a light beam passing through a first mirror, of the half reflective type, is reflected by a second mirror and then applied to a focusing detecting device. More particularly, the invention is directed to an improvement in the invention described in U.S. Pat. No. 4,342,508 or Japanese Utility Model application No. 128629/1980 (Japanese Utility Model application Laid-Open No. 51940/1982) by the present applicant.
In the aforementioned application, a first mirror holding frame is rotatably supported by a mirror box body, and a second mirror holding frame having a second mirror is rotatably mounted on the first mirror holding frame. When in an observation position a light beam reflected by the second mirror is applied to a focusing detecting device. When in a photographing position, a second mirror operating lever lifts the second mirror holding frame in association with the rise of the first mirror holding frame. It is essential that the focusing detecting device be set at the position which is in conjugation with the focusing screen of the finder and the surface of the film, and for the light beam to be perpendicular to the focusing detecting device. In the aforementioned application, the angle of opening of the second mirror holding frame can be adjusted by an angle adjusting pin. However, with this pin as the only adjusting means, it is sometimes impossible to make the incident light beam perpendicular to the focusing detecting device because of errors in components dimensions, or the like. The focusing detecting range of the focusing detecting device is displayed on the finder's focusing screen, for instance, as a central horizontal line on a split image type focusing screen, or as a U-shaped pattern or an inverted-U-shaped pattern. However, if the focusing detecting device is not movable, and there are dimensional errors in the components, the focusing detecting range displayed on the focusing screen does not coincide with the actual focusing range, and as a result the lens is not accurately focused on the aimed object.